Virtual packages are flagged as unused #231
Description
The tool flags virtual packages as unused dependency although they do not contain any code and therefore cannot be used anyway.
While this is not important in a regular project in libraries you often want to be able to tell the user that they have to add an implementation for a psr standard but (as it is a standard) you don't care which one.
Examples for virtual packages:
psr/http-client-implementationpsr/cache-implementationpsr/log-implementation
Therefore the composer dependency analyser tool should be able to ignore these virtual packages.